update vim documentation, add example for overriding sections.

doc/airline.txt

@@ -6,15 +6,23 @@ RATIONALE                                                          *airline*
 there's already powerline <https://github.com/Lokaltog/powerline>, why yet
 another statusline?
 
-*  it's standard vimscript, no python needed
-*  it's small. i want the entire plugin to be less than 200 lines as a rule
-*  gets you 90% of the way there: in addition to all the standard goodies,
-*  supports fugitve <https://github.com/tpope/vim-fugitive> and
-*  syntastic <https://github.com/scrooloose/syntastic>
-*  integration with vim-bufferline <https://github.com/bling/vim-bufferline>
-*  looks good with regular fonts, with support to use powerline font glyths
-*  it's fast to load.  since it's so small, it only takes 0.5ms to load.
-   by comparison, powerline needs 60ms on the same machine.
+*  it's 100% vimscript; no python needed.
+*  it's small.  i want the core plugin to be less than 200 lines as a rule.
+*  it gets you 90% of the way there; in addition to all the standard goodies,
+   it integrates with:
+  *  fugitve <https://github.com/tpope/vim-fugitive>
+  *  syntastic <https://github.com/scrooloose/syntastic>
+  *  vim-bufferline <https://github.com/bling/vim-bufferline>
+  *  unite <https://github.com/Shougo/unite.vim>
+  *  ctrlp <https://github.com/kien/ctrlp.vim>
+*  it looks good with regular fonts, and provides configuration points so you
+   can use unicode or powerline symbols.
+*  it's fast to load, taking roughly 1ms.  by comparison, powerline needs
+   60ms on the same machine.
+*  it's fully customizable; if you know a little statusline syntax you can
+   tweak it to your needs.
+*  it's trivial to write colorschemes; for a minimal theme you need to edit
+   9 lines of colors. (please send pull requests if you create new themes!)
 
 ==============================================================================
 NAME                                                          *airline-name*
@@ -119,6 +127,11 @@ with the usual statusline syntax.
   let g:airline_section_x      (filetype)
   let g:airline_section_y      (fileencoding, fileformat)
   let g:airline_section_z      (percentage, line number, column number)
+
+  " here is an example of how you could replace the fugitive indicator with
+  " the current working directory, followed by the filename.
+  let g:airline_section_b = '%{getcwd()}'
+  let g:airline_section_c = '%t'
 <
 
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